1. Field of the Invention
The present invention relates to an exposure apparatus suitable for manufacturing a device, such as a semiconductor device, and a device manufacturing method using the same.
2. Description of the Related Art
An exposure apparatus used to manufacture a semiconductor device projects a pattern of an original onto a substrate, coated with a photosensitive agent, via a projection optical system, to expose the photosensitive agent. The exposure apparatus may be of a step and repeat scheme or a step and scan scheme.
FIG. 1 is a view showing a schematic arrangement of an exposure apparatus. The following description is based on an X-Y-Z orthogonal coordinate system shown in FIG. 1. In the exposure apparatus, an original (reticle) R and a substrate (wafer) W are arranged at almost optically conjugate positions via a projection optical system UL. An illumination optical system IL illuminates, of the entire original R, an arcuate area or a slit-like area having a long side in the X direction. An original stage RS and a substrate stage WS are driven at a speed ratio corresponding to the optical magnification of the projection optical system UL in a direction perpendicular to the optical axis (Z-axis) of the projection optical system UL, that is, in the Y direction. The pattern of the original R is transferred onto the substrate W with exposure light emitted by a light source LS.
A control system, which includes a laser interferometer 101 for measuring the position of the original stage RS and an actuator for driving the original stage RS, controls the position of the original stage RS in the Y direction. In addition to the original R, an original reference plate RP is arranged on the original stage RS. The height of the pattern surface of the original reference plate RP almost coincides with that of the original R. A plurality of position measurement marks, made of a metal, such as Cr, are formed on the pattern surface of the original reference plate RP. The original stage RS is driven, while its position in the Z direction is kept constant relative to the projection optical system UL. A moving mirror 102 for reflecting the light beam emerging from the laser interferometer 101 is fixed to the original stage RS. The laser interferometer 101 successively measures the position and amount of movement of the original stage RS. The original stage RS may be one that can be driven not only in the Y direction, but also, in the X and Z directions. In this case, the control system, including the laser interferometer and actuator, controls the position of the original stage RS in the X and Z directions.
A substrate reference plate WP is arranged on the substrate stage WS. The height of the pattern surface of the substrate reference plate WP almost coincides with that of the upper surface of the substrate W. A plurality of position measurement marks, made of a metal, such as Cr, are formed on the pattern surface of the substrate reference plate WP. The control system controls the substrate stage WS to be movable in the Z direction and within the X-Y plane, and to be finely rotatable about the θX-, θY-, and θZ-axes. Similar to the original stage RS, the moving mirror 102 for reflecting the light beam from the laser interferometer 101 is fixed to the substrate stage WS. The laser interferometer 101 successively measures the position of the substrate stage WS in the X, Y, and Z directions, and its amount of movement.
A surface position detector will be explained next. The exposure apparatus comprises a grazing-incidence (oblique-incidence) surface position detector 103 to detect the surface position on the substrate W. The surface position detector 103 includes an illumination unit 103a and a detection unit 103b. The illumination unit 103a emits a plurality of light beams to obliquely illuminate the surface of the substrate W, onto which the pattern of the original R is to be transferred via the projection optical system UL. The detection unit 103b detects the plurality of light beams reflected by the surface of the substrate W. A plurality of photo-receiving elements are arrayed on the detection unit 103b of the surface position detector 103 in one-to-one correspondence with the plurality of light beams. The photo-receiving surface of each photo-receiving element is set to be almost conjugate to the reflection point of the corresponding light beam, on the surface of the substrate W. The surface position detector 103 detects the positional shift of the substrate W in the Z direction as the positional shift of the light beam applied to the photo-receiving element of the detection unit 103b. 
The substrate stage WS is driven in the horizontal direction to cause the surface position detector 103 to measure the positional shift of the surface of the substrate W (or substrate reference plate WP) from its measurement origin. This positional shift results from “undulation” or “warp” of the substrate W (or substrate reference plate WP).
Ideally, the focal plane of the projection optical system UL coincides with the measurement origin of the surface position detector 103. In exposure, the substrate stage WS is driven in the Z direction to correct the positional shift of the surface of the substrate W from the measurement origin of the surface position detector 103, that is, the positional shift of the surface of the substrate W from the focal plane of the projection optical system UL. This correction driving allows exposure, while the surface of the substrate W is positioned at a focal plane FP of the projection optical system UL.
However, when the projection optical system UL absorbs heat generated by the exposure light or when the surrounding environment varies, a positional shift occurs between the measurement origin of the grazing-incidence surface position detector 103 and the focal plane of the projection optical system UL. To solve this problem, it is necessary to measure and to correct the positional shift via the projection optical system UL. This correction is called focus calibration. The focus calibration can use an image detection TTR (Through The Reticle) detector.
A TTR detector 104 is an enlargement optical system. One exposure apparatus can incorporate one or a plurality of TTR detectors 104.
The TTR detector 104 can include an illumination unit 105, an objective lens 106, a relay lens 107, an image sensor 108, and a light guide system 109. The light guide system 109 guides exposure light to the illumination unit 105. The light source of the TTR detector 104 need not always use exposure light, but may use non-exposure light. The TTR detector 104 can sense an original reference mark RM and a substrate reference mark WM, and also functions as a position detection means for detecting the relative positions of these reference marks. The detection of these relative positions is used for calculation (X-Y calibration) of the shift between the driving directions of the original stage RS and substrate stage WS. The TTR detector 104 is movable in the X and Y directions to be able to detect the reference mark at each image height of the projection optical system UL.
While being supported by, for example, an air bearing in a noncontact manner, the substrate stage WS moves on its surface plate processed with a very high accuracy in the Z direction. The driving area of the substrate stage WS widens along with an increase in the size of the substrate W. The depth of focus is becoming shallow along with the formation of a finer pattern to be transferred. Therefore, it is becoming more difficult to accurately process the substrate stage surface plate such that the entire driving area of the substrate stage WS satisfies a sufficiently large depth of focus.
A variation in load upon driving the substrate stage WS also deforms the substrate stage surface plate. It is also difficult to process, into a completely flat surface, the reflection surface of the moving mirror 102, referred to by the laser interferometer 101, in order to control the position of the substrate stage WS. Consequently, many moving mirrors 102 mounted in the exposure apparatus have an uneven reflection surface. From a long-term viewpoint, the shape of the reflection surface of the moving mirror 102 changes upon an environmental variation in the exposure apparatus, and also, depending on the held state of the moving mirror 102. For these reasons, even when the substrate stage WS is driven in the horizontal direction, it shifts in the Z direction. This shifts the surface of the substrate W in the Z direction.
Assume, for example, that the surface position detector 103 detects a surface position Zp of the substrate W to correct and to drive the substrate stage WS by an amount of Zp in exposure. Even in this case, a positional shift Zd upon the positional shift of the substrate stage WS deviates the surface position of the substrate W from the best-focus position FP of the projection optical system UL by an amount of Zd. To prevent this problem, it is necessary to reduce a focus error by measuring the positional shift of the substrate stage WS in advance and to correct it in driving the substrate stage WS.
To solve these problems, a conventional exposure apparatus measures the positional shift of the substrate stage WS in the Z direction by arranging two detection areas of the surface position detector 103 in the driving direction of the substrate stage WS. This measurement method will be explained with reference to FIG. 2. The surface position detector 103 includes first and second detectors for respectively measuring the surface positions of the substrate W in the two detection areas along the driving direction of the substrate stage WS. Let Ly be the interval between the two detection areas. The first and second detectors use first and second light beams 103-1 and 103-2, respectively.
The substrate W, generally, has an uneven surface. When the substrate W is located at a position Y=y0, the first detector measures the surface position of the substrate W at a point A with the first light beam 103-1. This measurement value is defined as Zp(y0). Assume that the substrate stage WS is driven in the Y direction by Ly and its coordinate changes from the position Y=y0 to a position Y=y1. The second detector measures the surface position of the substrate W at the point A with the second light beam 103-2. This measurement value is defined as Zp(y1). The first detector using the first light beam 103-1 and the second detector using the second light beam 103-2 detect the same point A. For this reason, these measurement values must be equal, irrespective of the unevenness of the surface of the substrate W. However, assuming that the surface of the substrate W shifts in the focus direction (Z direction) by an amount Zd upon driving the substrate stage WS, the detected measurement value Zp(y1) is shifted by an amount Zd, with respect to the measurement value Zp(y0). That is, we haveZp(y1)=Zp(y0)+Zd where Zp(y0) and Zp(y1) are the measurement values obtained by the first and second detectors. The above-described relational expression allows the calculation of the positional shift Zd.
In accordance with the above-described principle, the first and second detectors detect the surface position of the substrate W over its entire surface while driving the substrate stage WS. This makes it possible to measure the positional shift of the substrate stage WS in the Z direction over its entire movable range.
The measurement data thus obtained can be associated with the coordinates of the substrate stage WS and stored in a memory as a correction table. In exposure, the substrate stage WS is corrected and driven using the correction table stored in the memory. This correction driving makes it possible to accurately position the surface of the substrate W at the focal plane FP of the projection optical system UL.
To cope with an increase in the degree of integration of semiconductor devices, a stronger demand has arisen for forming a finer pattern to be transferred onto a substrate, that is, for increasing the resolution. Under the circumstances, the shortening of the exposure wavelength has limitations. These days, in addition to the wavelength shortening, an attempt to meet this demand is made by increasing the numerical aperture (NA) of the projection optical system UL from about the conventional 0.6 to more than 0.9. Moreover, there is proposed a liquid immersion exposure apparatus in which part of the space between the projection optical system UL and the substrate W is filled with a liquid having a refractive index higher than one to increase the NA, thereby forming a finer exposure pattern.
In the liquid immersion exposure apparatus, the space between the substrate W and the optical element, which forms the projection end side (imaging plane side) of the projection optical system UL, is filled with a liquid having a refractive index close to that of the resist layer. This makes it possible to increase an effective NA of the projection optical system UL, when seen from the side of the substrate W, to result in an improvement in resolution. The liquid immersion projection method is expected to achieve good imaging performance by optimally selecting a liquid to be used.
In a high-NA exposure apparatus, it is necessary to arrange the substrate W and a lens (end lens) of the projection optical system UL, which is nearest to the substrate W, adjacent to each other, to suppress an increase in the size of the end lens. A liquid immersion exposure apparatus requires arranging the substrate W and the end lens of the projection optical system UL adjacent to each other, also from the viewpoint of stably holding the liquid between the projection optical system UL and the substrate W. In such an arrangement, it is impossible to arrange the surface position detector 103 around the projection optical system UL, so that the exposure area on the substrate W coincides with the measurement area of the surface position detector 103.
An improvement in optical design, such as the widening of an angle at which detection light of the surface position detector 103 is incident on the surface of the substrate W, or the decrease in the NA of the detection light, may be a countermeasure. However, this countermeasure causes a decrease in the size of the surface position detector 103 and a shortage of the amount of light, to result in a significant deterioration in detection accuracy.
To solve this problem, it may be a countermeasure to arrange the surface position detector 103 at a position where a necessary accuracy can be maintained, that is, a position spaced apart from the projection optical system UL, to set a position different from the exposure area on the substrate W as the measurement area of the surface position detector 103. However, even when this method is used to generate a correction table for the positional shift of the substrate stage, the position of the moving mirror 102 referred to by the laser interferometer in generating the correction table becomes different from that in controlling the substrate stage during exposure. Furthermore, the position of the substrate stage WS on its surface plate changes between the correction table generation time and the exposure time. This makes it impossible to accurately position the substrate W at the focal plane FP of the projection optical system UL, even by correcting and driving the substrate stage WS based on the correction table obtained, while the measurement area of the surface position detector 103 does not exist in the exposure area.